Full Idea
All the discourses of mathematicians concerning conic sections etc. concern not the existence of any of those figures, but their demonstrations, which depend on their ideas, are the same, whether there be any square or circle existing in the world or no.
Gist of Idea
Mathematics is just about ideas, so whether circles exist is irrelevant
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.04.08)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.566
A Reaction
If the full-blown platonic circle really existed, we would have the epistemic problem not only of getting in causal contact with it, but also of knowing whether our idea of it was the correct idea. We can't know that, so we just work with our idea.